Vincenzo Lordi

Young’s modulus of single-walled carbon nanotubes

We report in detail that unlike other materials, carbon nanotubes are so small that changes in structure can affect the Young’s modulus. The variation in modulus is attributed to differences in torsional strain, which is the dominant component of the total strain energy. Torsional strain, and correspondingly Young’s modulus, increases significantly with decreasing tube diameter and increases slightly with decreasing tube helicity.

Molecular mechanics of binding in carbon-nanotube–polymer composites

Nanoscale composites have been a technological dream for many years. Recently, increased interest has arisen in using carbon nanotubes as a filler for polymer composites, owing to their very small diameters on the order of 1 nm, very high aspect ratios of 1000 or more, and exceptional strength with Youngs modulus of approximately 1 TPa. A key issue for realizing these composites is obtaining good interfacial adhesion between the phases. In this work, we used force-field based molecular mechanics calculations to determine binding energies and sliding frictional stresses between pristine carbon nanotubes and a range of polymer substrates, in an effort to understand the factors governing interfacial adhesion. The particular polymers studied were chosen to correspond to reported composites in the literature. We also examined polymer morphologies by performing energy-minimizations in a vacuum. Hydrogen bond interactions with the ∏-bond network of pristine carbon nanotubes were found to bond most strongly to the surface, in the absence of chemically altered nanotubes. Surprisingly, we found that binding energies and frictional forces play only a minor role in determining the strength of the interface, but that helical polymer conformations are essential.

Structure and Oxidation Patterns of Carbon Nanotubes

We discuss the oxidation of carbon nanotubes and how it is affected by structure and geometry. While graphite is known to oxidize primarily at defects to create etch pits, nanotubes have additional structural features such as high curvature, helicity, and contain five and seven membered rings which modify the initiation and propagation of oxidation. Oxidation does not necessarily start at the tip of the tubes, and there are pronounced differential oxidation rates between layers which depend on the helicity of the individual shells.

Low-threshold continuous-wave 1.5-/spl mu/m GaInNAsSb lasers grown on GaAs

We present the first continuous-wave (CW) edge-emitting lasers at 1.5 /spl mu/m grown on GaAs by molecular beam epitaxy (MBE). These single quantum well (QW) devices show dramatic improvement in all areas of device performance as compared to previous reports. CW output powers as high as 140 mW (both facets) were obtained from 20 /spl mu/m /spl times/ 2450 /spl mu/m ridge-waveguide lasers possessing a threshold current density of 1.06 kA/cm/sup 2/, external quantum efficiency of 31%, and characteristic temperature T/sub 0/ of 139 K from 10/spl deg/C-60/spl deg/C. The lasing wavelength shifted 0.58 nm/K, resulting in CW laser action at 1.52 /spl mu/m at 70/spl deg/C. This is the first report of CW GaAs-based laser operation beyond 1.5 /spl mu/m. Evidence of Auger recombination and intervalence band absorption was found over the range of operation and prevented CW operation above 70/spl deg/C. Maximum CW output power was limited by insufficient thermal heatsinking; however, devices with a highly reflective (HR) coating applied to one facet produced 707 mW of pulsed output power limited by the laser driver. Similar CW output powers are expected with more sophisticated packaging and further optimization of the gain region. It is expected that such lasers will find application in next-generation optical networks as pump lasers for Raman amplifiers or doped fiber amplifiers, and could displace InP-based lasers for applications from 1.2 to 1.6 /spl mu/m.

Lithium Ion Solvation and Diffusion in Bulk Organic Electrolytes from First-Principles and Classical Reactive Molecular Dynamics

Lithium-ion battery performance is strongly influenced by the ionic conductivity of the electrolyte, which depends on the speed at which Li ions migrate across the cell and relates to their solvation structure. The choice of solvent can greatly impact both the solvation and diffusivity of Li ions. In this work, we used first-principles molecular dynamics to examine the solvation and diffusion of Li ions in the bulk organic solvents ethylene carbonate (EC), ethyl methyl carbonate (EMC), and a mixture of EC and EMC. We found that Li ions are solvated by either carbonyl or ether oxygen atoms of the solvents and sometimes by the PF6(-) anion. Li(+) prefers a tetrahedrally coordinated first solvation shell regardless of which species are involved, with the specific preferred solvation structure dependent on the organic solvent. In addition, we calculated Li diffusion coefficients in each electrolyte, finding slightly larger diffusivities in the linear carbonate EMC compared to the cyclic carbonate EC. The magnitude of the diffusion coefficient correlates with the strength of Li(+) solvation. Corresponding analysis for the PF6(-) anion shows greater diffusivity associated with a weakly bound, poorly defined first solvation shell. These results can be used to aid in the design of new electrolytes to improve Li-ion battery performance.

RADIAL COMPRESSION AND CONTROLLED CUTTING OF CARBON NANOTUBES

Utilizing high-resolution transmission electron microscopy, in tandem with molecular dynamics simulations, we conduct a fundamental study of the mechanical force–structure relationship of carbon nanotubes. We study the response of tubes to asymmetrical radial compressive forces, which are directly related to observed structure distortions in HRTEM (high-resolution transmission electron microscopy). We show that the magnitude of such forces can be readily estimated and is related to the shape of the distortion, characterized by percent compression and the radius of the tube, as well as the number of layers for very narrow tubes. The elasticity and resilience of the walls depend on the tube radius and the number of layers. We further suggest that even very large forces produce reversible, elastic distortions, indicating that radial mechanical forces should not be a feasible method to cut a nanotube without causing severe structural damage.

Extrinsic point defects in aluminum antimonide

We investigate thermodynamic and electronic properties of group IV (C, Si, Ge, Sn) and group VI (O, S, Se, Te) impurities as well as P and H in aluminum antimonide (AlSb) using first-principles calculations. To this end, we compute the formation energies of a broad range of possible defect configurations including defect complexes with the most important intrinsic defects. We also obtain relative carrier scattering strengths for these defects to determine their impact on charge carrier mobility. Furthermore, we employ a self-consistent charge equilibration scheme to determine the net charge carrier concentrations for different temperatures and impurity concentrations. Thereby, we are able to study the effect of impurities incorporated during growth and identify optimal processing conditions for achieving compensated material. The key findings are summarized as follows. Among the group IV elements, C, Si, and Ge substitute for Sb and act as shallow acceptors, while Sn can substitute for either Sb or Al and displays amphoteric character. Among the group VI elements, S, Se, and Te substitute for Sb and act as deep donors. In contrast, O is most likely to be incorporated as an interstitial and predominantly acts as an acceptor. As a group V element, P substitutes for Sb and is electrically inactive. C and O are the most detrimental impurities to carrier transport, while Sn, Se, and Te have a modest to low impact. Therefore, Te can be used to compensate C and O impurities, which are unintentionally incorporated during the growth process, with minimal effect on the carrier mobilities.

Red light-emitting diodes based on InP∕GaP quantum dots

The growth, fabrication, and device characterization of InP quantum-dot light-emitting diodes based on GaP are described and discussed. The diode structures are grown on gallium phosphide substrates using gas-source molecular-beam epitaxy and the active region of the diode consists of self-assembled InP quantum dots embedded in a GaP matrix. Red electroluminescence originating from direct band-gap emission from the InP quantum dots is observed at low temperatures.With increasing temperature, however, the emission line shifts to the longer wavelength. The emission light is measured to above room temperature.

Quantum-confined Stark effect of GaInNAs(Sb) quantum wells at 1300–1600nm

We report the measurement of electroabsorption spectra from GaInNAs and GaInNAsSb quantum wells grown on GaAs showing quantum-confined Stark effect behavior suitable for optical modulation at 1300 and 1550nm wavelength, respectively. The high quality of our material is evidenced by sharp exciton resonances with a full width at half maximum <25meV at 295K, and peak absorption coefficient of 18 000cm−1 for GaInNAs and 34 800cm−1 for GaInNAsSb. Changes in absorption coefficient 10 000cm−1 with an applied electric field were measured. Device performance from these materials is expected to be comparable to or better than the competing material grown on InP.

Neutron detection with single crystal organic scintillators

Detection of high-energy neutrons in the presence of gamma radiation background utilizes pulse-shape discrimination (PSD) phenomena in organics studied previously only with limited number of materials, mostly liquid scintillators and single crystal stilbene. The current paper presents the results obtained with broader varieties of luminescent organic single crystals. The studies involve experimental tools of crystal growth and material characterization in combination with the advanced computer modeling, with the final goal of better understanding the relevance between the nature of the organic materials and their PSD properties. Special consideration is given to the factors that may diminish or even completely obscure the PSD properties in scintillating crystals. Among such factors are molecular and crystallographic structures that determine exchange coupling and exciton mobility in organic materials and the impurity effect discussed on the examples of trans-stilbene, bibenzyl, 9,10- diphenylanthracene and diphenylacetylene.